Eid al-Ghadeer, observed by Shia Muslims on the 18th of Dhu al-Hijjah in the Islamic calendar, is a celebration of the appointment of Ali ibn Abi Talib by the Islamic prophet Muhammad as his successor. The festival also marks the anniversary of Muhammad's sermon, as described in Hadith of the pond of Khumm in which Muhammad states, "Whomsoever is protector (maula) of me, Ali is also his (her) protector (maula). O God, love those who love him, and be hostile to those who are hostile to him."
Several months before his death, Muhammad (who was living in Medina) made his last pilgrimage to Makkah in what is known as The Farewell Pilgrimage. There, atop Mount Arafat, he addressed Muslim crowds in what came to be known as The Farewell Sermon. After completing the Hajj, Muhammad started back towards Medina. During his trip, he stopped at the pond at Khumm (now al-Johfa) and appointed Ali as the master of believers. Muhammad also entered a covenant about his mastership with those present. The meaning of maula (master) is controversial among the major Islamic sects.
The Prophet’s sermon of Ali's succession was:
O people! Reflect on the Quran and comprehend its verses. Look into its clear verses and do not follow its ambiguous parts, for by Allah, none shall be able to explain to you its warnings and its mysteries, nor shall anyone clarify its interpretation, other than the one that I have grasped his hand, brought up beside myself, [and lifted his arm,] the one about whom I inform you that whomever I am his master (Maula), this Ali is his master (Maula); and he is Ali Ibn Abi Talib, my brother, the executor of my will (Wasiyyi), whose appointment as your guardian and leader has been sent down to me from Allah, the Greatest, Mighty and the Majestic.